The opnsense-update utility offers combined kernel and base system upgrades
using remotely fetched binary sets, as well as package upgrades via pkg.
For a complete list of options look at the manpage on the system.

Here you can see all the kernels for version 18.1. Be aware to change the version if you driver a newer version.
As an example you updated from 18.1.4 to 18.1.5 you have now installed kernel-18.1.5.
To revert back to the last stable you can see kernel-18.1 so the syntax would be:

# opnsense-update -kr 18.1

# /usr/local/etc/rc.reboot

Where -k only touches the kernel and -r takes the version number.

To switch back to the current kernel just use

# opnsense-update -k

# /usr/local/etc/rc.reboot

Warning

Before reverting a kernel please consult the forums or open an issue via Github.
You should only revert kernels on test machines or when qualified team members advise you to do so!

The opnsense-revert utility offers to securely install previous versions of packages
found in an OPNsense release as long as the selected mirror caches said release.
For a complete list of options look at the manpage on the system.

The opnsense-patch utility treats all arguments as upstream git repository commit hashes,
downloads them and finally applies them in order.
Patches can also be reversed by reapplying them, but multiple patches must be given in reverse order to succeed.
For a complete list of options look at the manpage on the system.